Geophysical electromagnetic (“EM”) techniques can be effective in determining the electrical conductivity of soils, rocks and other conductive material at depths from the surface of up to about three kilometers. Conductivity distribution at depths is of great interest to those involved in mapping base metals and uranium deposits, aquifers and other geological formations.
Geophysical EM methods can involve measurements of time-varying magnetic fields near the earth's surface. These may be produced by way of a primary magnetic field and modeling of the ground conductivity distributions. A magnetic field may be generated either by a periodic current applied to a transmitter, or by naturally occurring electromagnetic fields originating mainly from lightning in the earth's atmosphere. EM fields can have a characteristic ground penetration depth proportional to the inverse of the square-root of both ground conductivity and frequency.
Traditionally the magnetic field signal can be measured using either a receiver coil system (which can measure up to three orthogonal components of the magnetic field time-derivative dB/dt), or a magnetometer (which measures the magnetic field B). The received analog signal may then be amplified, filtered, and digitized by a high-resolution high-speed analog-to-digital converter (ADC), and the data can be stored along with the positioning information obtained from a Global Positioning System (GPS). Data post-processing may involve electrical and physical modeling of the ground to generate the geophysical conductivity contour maps.
Geophysical surveys may typically require high signal-to-noise ratio (SNR), high conductivity discrimination, and high spatial resolution, both laterally and in depth. The preferred EM method may differ depending on ground conductivity and the desired probing depth.
EM systems may encompass both ground-based and airborne measurements using airplanes and helicopters. Airborne methods may be preferred for large area surveys and can be used for exploration of conductive ore bodies buried in resistive bedrock, geological mapping, hydrogeology, and environmental monitoring.
For airborne electromagnetic (“AEM”) systems, the data may be acquired while the airplane or helicopter flies at a nearly constant speed (up to 75 m/s or 30 m/s, respectively) along nearly-parallel and close to equally-spaced lines (50 m to 200 m) at an approximately constant height above ground (about 120 m or 30 m, respectively). Measurements can be taken at regular intervals, typically in the range 1 m up to 100 m.
Fixed-wing airplane AEM systems may carry large transmitter coils and can be capable of deeper investigation of discrete conductors, such as base metals and uranium deposits, while helicopter frequency-domain electromagnetic (“HFEM”) systems can be effective in near-surface mapping with high near-surface resolution, although limited depth penetration. Over the last decade, the key driver in geophysical surveying has been to develop helicopter-mounted time-domain electromagnetic (“HTEM”) systems.
EM measurements can be recorded either in the frequency domain or time domain. In frequency-domain electromagnetic (“FDEM”) measurements, the transmitter coil continuously may transmit an electromagnetic signal at fixed multiple frequencies, while the receiver coil measures the signal continuously over time. The measured quantities may be either signal amplitude and phase as a function of frequency, or equivalently, the in-phase and in-quadrature amplitudes as a function of frequency. In these measurements, the signal sensitivity can be reduced with increasing conductivity, and thereby may reduce the conductivity contrast mapping.
In time-domain electromagnetic (“TDEM”) measurements, a pulse of current may be applied to the transmitter coil during an on-period and switched off during the off-period, typically at a repetition rate equal to an odd multiple of half of the local power line frequency (typically 5 GHz or 60 Hz). The signal may be measured at the receiver as a function of time. The signal amplitude decay during the off-period, combined with modeling of the conductivity and geometry of geological bodies in the ground, can be utilized to yield the conductivity contour maps.
In Audio Frequency Magnetic (“AFMAG”) measurements, naturally occurring EM fields produced by global lightning discharges may be used as the excitation source. These EM fields propagate around the earth as plane waves guided by the ionosphere and earth's surface. Lightning activity occurring more than 1000 km away from the measurement point can produce signals with a nearly flat spectral density between 8 Hz and 500 Hz, varying with geographical location, time of the day, seasons and weather conditions. Lightning occurring within about 1000 km from the measurement point can produce pulses with duration of a few milliseconds with spectral density in the range from 2 kHz to 20 kHz.
In AFMAG, the vertical component of the signal may be measured by the airborne receiver coil in the frequency range 25 Hz to 2 kHz with data acquisition at 6.25 kHz with 24 bits resolution. The measured signal may then be selected by frequency bands linearly spaced in the log scale spaced by approximately 1.5 dB, and then processed to produce conductivity contour maps. AFMAG may also uses measurements of the horizontal magnetic field in real time in order to normalize the measurements done in the aerial survey as the source intensity is constantly varying.
One possible AFMAG setup is to use two orthogonal coils at the ground base station to yield the horizontal component of the magnetic field, and one flying coil to measure the vertical component of the magnetic field. A third vertical coil can also be used at the ground base station to improve the measurement of the reference signal. The aerial measurements can be made typically at a distance less than 50 km from the ground base station.
Alternative setups can use three-component measurements at the ground station and/or three component measurements obtained in flight. Multiple base stations can also be used to locate the EM field source and improve the SNR of the measurements.
Besides the HTEM system provided by Geotech, named VTEM (Versatile Time-Domain Electromagnetic), over the last few years other systems became operational. These include: AeroTEM by Aeroquest Ltd., THEM by THEM Geophysics Inc., HoisTEM by Normandy Exploration Ltd., NewTEM by Newmont Mining Corp., ExploHEM by Anglo American, SkyTEM by SkyTEM ApS., and HeliGEOTEM by Fulgro Airborne Survey.
Whenever measurement of the signal proportional to dB/dt is required, a coil may be the best choice as sensor because it measures dB/dt directly. The voltage induced in the receiver coil by a magnetic field B is given by N.A.dB/dt, where the coil sensitivity N.A is the product of the coil number of turns N and the coil area A, and dB/dt is the time-derivative of the magnetic field. The inductance of a coil is proportional to N2.D, where N is the number of turns and D is the effective diameter of the coil.
For the receiver coil system of both the TDEM and AFMAG systems, the dynamic range required for the pre-amplifier and the ADC may typically be in the range of 120 dB, dictated by the ratio between the maximum signal amplitude and the input noise of the pre-amplifier.
In TDEM, to increase the dynamic range, one can place the receiver coil several meters above the transmitter coil to reduce the signal produced by the transmitter coil at the receiver. Alternatively, one can either use an auto-scaling pre-amplifier or switch the pre-amplifier gain between low-gain during the on-period and high-gain during the off-period. Use of adjustable gain amplifiers can make data acquisition more complex, but may have the advantage of keeping the transmitter and receiver coils concentric, thereby minimizing anomalous mapping profiles. For a 40 dB adjustable gain pre-amplifier, a 16-bit ADC is sufficient to digitize the signal. If a 24-bit ADC is used, the system may use a fixed gain pre-amplifier.
Sources of electrical noise at the receiver coil can include the spurious signals produced by both the helicopter and other metallic parts of the system, lightning activity in the atmosphere, local AC power line interference, VLF radio waves in the 15-25 kHz frequency range, and thermal noise from the coil and the electronics. However, the main source of noise at the airborne receiver coil can be the microphonic noise produced by the motion of the coil in the magnetic field of the earth. The motion can be produced by wind buffeting the coil, vibration from the aircraft, and rubbing of the coil against the coil suspension system.
In an attempt to increase SNR, U.S. Pat. No. 6,876,202 titled “System, Method and Computer Product Geological Surveying Utilizing Natural Electromagnetic Fields”, inventors Edward Beverly Morrison and Petr Valcntinovich Kuzmin, granted 5 Apr. 2005 discloses a receiver coil and suspension means that facilitates a reduction of microphonic noise produced by mechanical vibrations of the receiver coil in the magnetic field of the earth. The method applied by U.S. Pat. No. 6,876,202 is to surround the coil with an acoustic noise absorber. It also discloses a means of reducing noise through permitting distance between the sensors and the aircraft. However, the Morrison invention does not introduce with suspension mechanisms to mitigate these noises.
Furthermore U.S. Pat. No. 7,157,914 discloses a double-structure receiver suspension apparatus. This prior art invention is aimed at reducing vibration and microphonic noise, however the elastic suspension means disclosed is prone to noise created by the rubbing of elements of the receiver coil. There is need for a double-structure receiver suspension apparatus that further reduces such noise. In addition, there is a need for a double structure receiver suspension apparatus that ameliorates the maintenance of the receiver coil within the receiver frame in a generally horizontal position during flight.
Increasing signal-to-noise ratio (SNR) at the receiver coil may not be straightforward due to many factors affecting the measurement. In order to minimize the noise produced by various sources in the frequency range of interest, one may need to reduce the movement of the receiver coil relative to the magnetic field of the earth, prevent external mechanical noises from reaching the receiver coil, and minimize the mechanical noises produced by the receiver coil suspension system.
An advantage of the double-suspension receiver coil of the present invention is that it can be used to overcome the SNR problems of the prior art.